Use of a Constrained Mixture Design to Optimize a Procedure for Closed Vessel Microwave-Assisted Digestion of Vegetal Leaves

Background: Reliable analytical methods for metals determination in tea is a demand of the nutritional and toxicological sciences. In this work, a constrained mixture design has been applied to develop an efficient closed-vessel microwave-assisted digestion procedure for Ca, Cu, Fe, K, Mg, Mn, Na, Sr and Zn determination in vegetal leaves used for tea preparation. Methods: The optimization step was carried out using a constrained mixture design to find digested samples of good characteristics such as clarity, low residual carbon and low final acidity. The studied metals and residual carbon were determined by simultaneous inductively coupled plasma optical emission spectrometry (ICP OES). Results: The optimum proportions found by applying the mixture design were 31.3, 56.2 and 12.5% (v/v) for H2O, HNO3 and H2O2, respectively. The accuracy of the method was confirmed by analysis of two certified reference materials: apple leaves (NIST 1515) and spinach leaves (NIST 1570a). The proposed method was applied for the determination of studied metals in vegetal leaves used for the preparation of infusions. Conclusion: The developed method has presented adequate analytical characteristics to determining of studied elements from tea samples with quickly, accuracy, precision and reliability.